This invention relates to capacitance sensitive switches and capacitance sensitive switch arrays.
Capacitance sensitive switches currently exist in the art. They generally can be classified as either relating to frequency shift detecting circuits, to voltage divider circuits or to Resistance-Capacitance (RC) timing circuits.
Frequency shift detecting circuits are disclosed in U.S. Pat. No. 4,495,485 to Smith and U.S. Pat. No. 4,567,470 to Yoshikawa et al. Smith employs a network of capacitive devices which include sensing areas on a keyboard. The network presents a equivalent capacitance to an RC circuit which ultimately determines an oscillation frequency of an oscillator employing a 555 timer. Detection of the shift in oscillating frequency of the 555 timer is used to determine whether or not an object is in proximity to a sensing area.
U.S. Pat. No. 4,567,470 to Yoshikawa et al., discloses sensing plates connected to an analogue multiplexer which ultimately connects each plate to an oscillator circuit. A compensating circuit is connected to the multiplexer to compensate each plate capacitance to cause the oscillator to oscillate at the resonance frequency when no plate is touched. A comparator senses the average voltage level at an output of the oscillator. When a plate is touched, body capacitance shifts the frequency of oscillation causing a drop in voltage level which is detected by the comparator. The comparator gates the loading of registers with an address value representing the particular plate touched. Generally, the complexity of the circuitry required to detect changes in frequency due to capacitance is high in the above two devices.
Furthermore, there appears to be no ability to quickly and easily recalibrate the reference frequency of oscillation without modifying these circuits.
Voltage divider circuits are disclosed in U.S. Pat. Nos. 5,012,124 to Hollaway, 4,291,303 to Cutler et al., 4,145,748 to Eichelberger et al. and 4,772,874 to Hasegawa. The patent to Hollaway discloses a plurality of plates which are all activated at the same time by a common oscillator signal. A voltage divider is formed by a fixed resistor and capacitance of a human body touching a plate, creating a drop in voltage which is rectified and filtered to a DC value which is converted into a digital form. A high or low DC value relative to a previously stored reference value indicates the on/off condition of a signal line representing the condition of the switch. The signal lines are multiplexed and interpreted by a microprocessor. The external multiplexer and analog to digital converter add unnecessary complexity and cost to this circuit.
The patent to Cutler et al., discloses a plurality of capacitive coupling devices which couple a signal through the capacitance of a user's finger to signal ground. The patent states that the scan signal voltage is reduced by about 20% due to such coupling. This drop in voltage is buffered and applied to an RC circuit which produces an output voltage which is compared to a threshold voltage and is used to trigger a comparator if the output voltage is below the threshold voltage, as would be the case if a plate were touched. Additional circuitry is required to overcome the effects of coupling the capacitance of the user's finger into more than one coupling device.
The patent to Eichelberger et al., discloses a capacitive keyboard array including a plurality of key plates which couple the capacitance of an operator's finger to signal ground to effect a voltage divider. The effect is that a signal having a voltage level dependent on the proximity of the user's finger to the plate is created and supplied to an analog to digital (A/D) converter which compares a digital value representing the signal to a previously stored reference value. The output of the comparator indicates the touched and untouched states of the plate. The reference value is periodically updated by incrementing or decrementing a counter into which the reference value is loaded. This suggests additional circuitry.
The patent to Hasegawa discloses a keyboard which has depressible keys. The keys capacitively couple a clock signal from a column line to a row line and produce a signal on the row line having a pulse width defined by the time between pressing and releasing the key. The greater the depth of depression of a key, the greater the pulse width. Detection of the pulse width is used to determine whether or not a key is depressed. Thus a user must keep a key depressed for a required period of time in order to activate a given key.
Resistance-Capacitance timing circuits are disclosed in U.S. Pat. No. 4,595,913 to Aubuchon and in U.S. Pat. No. 4,157,539 to Hunts et al. Aubuchon discloses a plurality of asynchronous sensing circuits, each operable to receive a clock pulse from a common clock. Each sensing circuit receives and inverts the clock pulse to present a low voltage level to an RC network, the C of which is formed by the user's finger. The voltage level of a digital signal produced by a NAND gate connected to the RC network is then clocked into a D flip-flop after a pre-specified or reference time, the output of the D flip-flop representing true and false states of the touched condition.
The patent to Hunts et al., discloses a plurality of coupling devices connected between signal lines. Proximity of the user's finger to a plate couples both signal lines to ground. Each of the signal lines is then set high sequentially which initiates a charging cycle in a first RC network including the user's finger and in a second RC network including a reference capacitor. Depending upon which RC network achieves a desired voltage level first, a flip-flop is set to indicate the presence or absence of the user's finger on the plates corresponding to the actuated signal lines.
The problem with the above RC timing devices is that they each employ a common reference capacitance for comparison with a number of switch units. Consequently, some switch units may be more sensitive than others due to variances in capacitance caused by circuit board layout. This can have adverse effects on the ability to detect objects in physical proximity with the sensing means and false activation of the switch is more likely.
What would be desirable is a capacitance sensitive switch array in which each switch unit is individually selectable and in which a digital timer is used to produce a timer value indicating the time taken to charge the capacitance of a selected switch unit and in which the digital timer value is compared to a corresponding digital reference timer value representing the timer value produced by the selected switch unit when no external influence is applied, to produce a signal indicating the on/off condition of the switch unit. In this manner each switch unit can have a separate reference timer value.